1. Field of the Invention
This invention relates to retrieval of magnetically recorded information by optical means using the property of a magnetic field to rotate the plane of polarization of a beam of coherent light. This property is commonly known as the Faraday Effect where the light is transmitted through the recording medium and the Kerr Effect where the light is reflected therefrom.
2. Description of the Prior Art
A technique for retrieving magnetically recorded information by use of the Kerr or Faraday Effects is disclosed in U.S. Pat. No. 3,284,785, issued to Kornei. In the apparatus described by Kornei, a "rotator" (driven by a fixed frequency signal) periodically oscillates the direction of polarization of a beam of coherent polarized light which has been reflected from a magnetic recording medium. A phase detector measures the difference between the phase of the signal driving the rotator and the phase of the oscillated light beam.
One problem with this approach is that optical non-homogeneities which are present in the recording medium also influence the phase difference which is measured. Such influence is particularly troublesome because the shifts in the direction of the polarized beam which result from the magnetic recording (Kerr or Faraday Effect shifting) are quite small. In fact, if severe optical non-homogeneities are present in the recording medium, relative phase changes in the reflected light beam caused by the magnetic recording may be masked entirely by noise resulting from optical modulation of the beam.